A current mirror is a ubiquitous building block in many analog circuits, finding wide applications in such circuits as amplifiers, biasing circuits, analog-to-digital circuits, and digital-to-analog circuits, to name just a few. A drawback of using a standard current mirror in modern process technologies is that the transistors used in the current mirror have a relatively low output impedance. With low output impedance transistors, the current level in a standard current mirror changes as a function of drain voltage, which in general is undesirable. A known method of overcoming this is to utilize a cascode current mirror.
For a cascode current mirror to work properly, it is preferable that the various transistors forming the cascode current mirror are operating in their saturation region. However, in low voltage process technologies in which the transistor threshold voltage is on the order of 100 mV to 200 mV, one or more transistors in a cascode current mirror may easily go out of saturation, thereby limiting the desired current mirroring characteristics of the cascode current mirror.
It is desirable to provide current mirror structures with the relatively high output impedance of a standard cascode current mirror, but suitable for process technologies utilizing low threshold voltage transistors.